Ventricular tachycardia (VT) and ventricular fibrillation (VF) are common causes of the 300,000 sudden deaths occurring in the United States each year. Most of these victims have associated heart disease, most commonly coronary artery disease. Populations consuming considerable quantities of fish and marine mammals have lower than expected mortality rates from coronary disease. Interventional and observational trials have indicated that fatty fish consumption decreases the death rate from coronary artery disease, in part by reducing the number of sudden deaths. Animal and tissue culture studies both support the hypothesis that these beneficial effects are from the anti-arrhythmic properties of n-3 long chained polyunsaturated fatty acids (eicosapentaenoic and docosahexaenoic acids). In this prospective, randomized double blinded trial, survivors of VT and VF with an implantable defibrillator will be randomized to supplementation with these n-3 polyunsaturated fatty acids or with a placebo. Adherence to the supplement will be assessed by measurements of plasma, red cell, and adipose tissue n-3 fatty acid concentrations. The primary outcome variable will be the incidence of recurrent VT or VF, but secondary variables will also be assessed using serial ICD assessment, correlation of the rhythms with the biochemical measurements of n-3 fatty acids, hospitalization rates and quality of life. The ICD is the best protection available to patients and can store rhythm electrograms which will allow documentation of rhythm endpoints. If the dietary supplementation with n-3 fatty acids demonstrates a reduction of VT and VF in humans at high risk, this would be a stimulus to apply this inexpensive and safe form of treatment to the large populations who are at increased risk of sudden death.